WASHINGTON — Stepsiblings and half-siblings are allowed, but not nieces or nephews. Sons- and daughters-in-law are in, but brothers- and sisters-in-law are not. Parents, including in-laws, are considered “close family,” but grandparents are not.

The State Department issued new guidelines Wednesday night to U.S. embassies and consulates on applying a limited travel ban against foreign visitors from six predominantly Muslim countries. Enforcement of the guidelines will begin at 8 p.m. Eastern on Thursday.

The guidelines followed the Supreme Court’s decision Monday to allow parts of the Trump administration’s revised travel ban to move forward, while also imposing certain limits, as the court prepares to hear arguments in October on the scope of presidential power over border security and immigration.

The court said the ban could not be imposed on anyone who had “a credible claim of a bona fide relationship with a person or entity in the United States.”

The meaning of “bona fide relationship” was not precisely explained, and the phrase has created much uncertainty for migrants and others seeking to travel to the United States from the six countries — Iran, Libya, Somalia, Sudan, Syria and Yemen — covered by the revised travel ban that President Donald Trump issued in March. (An earlier version of the ban included Iraq.)

The Trump administration has now made the definition explicit.

According to a diplomatic cable obtained by The New York Times, “close family” is “defined as a parent (including parent-in-law), spouse, child, adult son or daughter, son-in-law, daughter-in-law, sibling, whether whole or half. This includes step relationships.”

But it went on to state that “close family” does not include “grandparents, grandchildren, aunts, uncles, nieces, nephews, cousins, brothers-in-laws and sisters-in-law, fiancés and any other ‘extended’ family members.”

It is not clear how the administration arrived at the new definitions.

Under existing law, Americans may petition for immigration visas for “immediate relatives” — defined as the parents, spouses and children (under 21) of American citizens.

Omar Jadwat, director of the American Civil Liberties Union’s immigrants’ rights project, said Thursday the new guidelines troubled him, particularly as they could be read as creating arbitrary definitions of family relationships.

“Initial reports suggest that the government may try to unilaterally expand the scope of the ban — for example, by arbitrarily refusing to treat certain categories of familial relationships as ‘bona fide,’ ” he said. “These reports are deeply concerning.”